If you want to read Lovecraft, get a short story collection for sure. The problem is that if you read a lot of lovecraftian horror, one after the other, the constant 'indesrcribable' nature of the stories can get a bit samie so take another book or something else to do to break up the stories better.
If that seems like a lot of effort, check out House of Leaves by Danielewski. It's about a [clor=blue]house[/color] that is bigger on the inside than the outside, but that description doesn't do it justice. It ratchets up the creepy so slowly that you don't even notice it until the crescendo kicks you in the face... and then it keeps going. I haven't finishde erading it yet, but I am really enjoying it.
Other, non-horror book suggestions: I cannot over-reccomend anything by Terry Pratchett or Neil Gaiman, or Good Omens by both of them. An excellent fantasy novel would be Magician by Raymond E. Feist (all of his books are good but that one is the best kicking off point since it is the first book fo the series that defines the setting). If you are into sci-fi, Fallen Dragon by Peter Hamilton is excellent and I firmly believe that everyone needs to read Orson Scott Card's Ender's Game. Finally, anything by Matthew Reilly is great for an edge of your seat action book. I still don't understand why none of them have been turned into movies yet. They probably wouldn't do the books justice, but the movie would still be a crazy awesome rollercoaster of non-stop action.
Hopefully that helps.